The present invention relates to a method and an apparatus for the optical measurement of the pressure of a gaseous mixture.
Various optical methods for measuring the pressure of a gaseous mixture have already been proposed in the prior art. We shall first of all recall the principles on which such measurements are based.
Very diagrammatically, an atom may be considered as a number of negatively charged electrons in orbit around a positively charged nucleus. The radii of the orbits are not random. In fact, in order that the trajectory of an electron is stable, its distance from the nucleus, i.e. the radius of its orbit, must be such that its kinetic moment, the product of its quantity of movement and this radius, is in the form of n.h/2.pi., where n is a whole number and h is a constant, the so-called Planck's constant.
When an electron is located on a permitted orbit defined in this way, its state is stationary and it radiates no energy. There consequently corresponds to a given orbit an energy which is itself given.
On the other hand, if an electron passes from a permitted orbit of energy E.sub.1 to another permitted orbit of energy E.sub.2, it undergoes a variation of energy equal to E.sub.2 -E.sub.1. If E.sub.1 is greater than E.sub.2, the electron then emits a photon (basic particle of light) of energy -(E.sub.2 -E.sub.1) of which one can show that the frequency .nu..sub.e is such that: EQU .nu..sub.e =-(E.sub.2 -E.sub.1)/h (1)
Similarly, if E.sub.1 is less than E.sub.2, the electron then absorbs a photon of frequency .nu..sub.a, such that: EQU .nu..sub.a =(E.sub.2 -E.sub.1)/h (2)
The energy of the electrons, and thus of the atoms is thus quantified. It can adopt only certain discrete values and the atom absorbs or emits electromagnetic radiation when its energy passes from one level to another. In the absence of excitation, the atom is at its ground level where its energy is minimal.
Similar processes take place at the molecular level.
A molecule is a number of atoms whereof the electrons ensure cohesion. The energy of a molecule depends, apart from the electronic energy of its constituent atoms, on the displacement energy of the nuclei.
Two types of displacement may be envisaged: on the one hand vibrations, corresponding to variations of distance between the nuclei of the atoms, and on the other hand rotations, in which the molecule rotates about itself.
One can show that in this case also the energy levels are quantified. In fact one can state that, from the vibrational point of view, a molecule may be either in its ground state, or in an excited state. The various possible excited states correspond to discrete energy levels, that is to say that the energy cannot vary continuously. A vibrational transition, i.e. a transition from one energy level to another corresponding to two distinct vibrational states, occurs when the molecule absorbs or emits a photon in the infrared range, that is to say of a frequency of the order of 10.sup.12 to 10.sup.14 Hz corresponding to wavelengths of the order of 1 .mu.m to 100 .mu.m.
Similarly, each vibrational energy level is divided into a discrete set of rotational energy sub-levels, that is to say that a molecule in a given vibrational state may be in a certain number of different states from the rotational point of view. A rotational transition of the molecule, that is to say the passage of the molecule from one rotational sub-level to another, whilst preserving the same vibrational state, corresponds to the emission or absorption of a photon whereof the frequency is in the radio-frequency or far infrared range, i.e. of the order of 10.sup.6 to 10.sup.12 hertz, which corresponds to wavelengths varying from approximately 100 meters to approximately 100 micrometers.
As in the case of the electronic energy of an atom, the vibrational and rotational energy of a molecule may vary solely by predetermined quantities corresponding to the jump from one permitted energy level to another permitted energy level. The result is that the frequencies of photons which may be emitted or absorbed by a molecule have clearly determined values, given respectively by the formulae (1) and (2) above.
Corresponding to all the frequencies of the photons able to be emitted by a molecule is a number of emission lines constituting the emission spectrum of this molecule. A number of absorption lines constituting the absorption spectrum of the molecule corresponds similarly to the number of frequencies of the photons able to be absorbed by the molecule.
Lines close to each other may be grouped in bands which correspond to a certain range of frequencies.
Methods for the optical measurement of the pressure of a gaseous mixture are generally based on an excitation by absorption of the photons of a laser beam, of the molecules of one of the constituents of this mixture. The pressure is then obtained by observing the characteristics of the de-excitation process of the molecule, or relaxation, and by linking these characteristics with the pressure and other known physical parameters, in particular with the temperature.
Thus the document U.S. Pat. No. 5,111,055 describes a method for the remote optical measurement of the pressure of the air, in particular in front of an aircraft, in which there are excited, by means of a laser generator, transitions of energy bands of the so-called Schumann-Runge range of molecular oxygen.
A laser able to be used for these measurements is the laser with an argon-fluorine excimer able to be tuned in the ultra-violet range about a wavelength of 193 nanometers. However, available argon-fluorine lasers cannot be carried on board an aircraft and, in addition, the obtaining of a laser beam at 193 nanometers by the mixing of frequencies also requires very heavy means.